I can't see how you'd get it to fit under the vehicle when you're down to the door sills in mud. Single-stage hydraulic rams all create this problem - they are either too long to fit under when the vehicle is bogged down, or they are not long enough to lift high enough. So a 300mm jack (or piston) needs a minimum 300mm of space under the jacking point, and then raises the vehicle by a maximum of 300mm. A longer jack gives more lift, but needs more space.
Multi-stage hydraulics (pistons within pistons) might partially get around this problem in principle, but they start getting pretty vulnerable in lots of ways, and there are snags with the variable pumping ratio.
I like the thinking, though - for bigger trucks, what about using the hydraulic stabilisers they use on cranes? If one mounted suitable brackets on all four corners of the truck, one could use a pintle to position the ram as low as necessary, and then get a full piston-height worth of lift.
Rams don't generally like any sideways forces, so that might be a problem in real-life situations... Luke or someone will give a bottle of Champagne for the most elegant solution